Armor coatings and/or abradable bedding-in linings are provided on engine components such as on the tips of sealing fins of labyrinth seals or the tips of rotor blades, in order to hinder the abrasive wear of the coated components during grazing contact between the armored component and the other component provided with a bedding-in coating. Since the efficiency of a compressor or a turbine is largely dependent on the gap width between the rotating component and the stationary component, this efficiency is reduced with increasing abrasive wear, for example of the blade tips, as a result of grazing contact processes.
Generally, the armor coating abrades or cuts itself into an opposing bedding-in coating of a second component during operation of the engine. Such bedding-in coatings generally consist of a layer of an abradable, corrosion resistant and erosion resistant material. However, if the strength and hardness of the bedding-in coating is increased in order to increase the erosion and temperature resistance thereof, then the abrasive wear of the engine components will also be increased, so that it is necessary to provide an armor coating thereon. By providing such an armor coating, it is achieved that a minimized gap width will be formed between the armor coating and the bedding-in coating as a result of the grazing contact therebetween.
Known armor coatings comprise a ceramic layer that is thermally sprayed onto the surface of the respective engine component. In order to provide, on the armor coating, a profiled surface having edges or peaks that are adapted to cut into the opposing bedding-in coating, as well as free spaces or depressions between respective peaks for receiving and carrying away the abraded material, the known thermal spray process is carried out using a perforated mask consisting of a wire mesh arranged over the component surface. Then the coating material is sprayed through the wire mesh mask, whereby the spray is selectively blocked in a pattern determined by the mask so as to form peaks and depressions in the spray coating applied to the engine component. The size and geometry of the respective peaks or edges is determined and may be varied by the position and type of wire mesh that is used as the mask, as well as the spraying stream angle and the like.
In practice, this known process has been found to suffer several disadvantages. First, it has been found that the wire mesh completely covers the component surface or completely blocks the spray application at certain locations and thereby causes the formation of defects, gaps or voids in the coating layer. This also causes undesirable variations in the coating layer thickness, and makes it impossible to achieve a uniform surface profiling over the entire circumference of the component. Furthermore, the known process requires a complicated preparation of the component surface as well as handling of the wire mesh, and involves difficulties in maintaining the required spraying stream angle and limiting that angle to the range specified by the known process.